The present invention relates to a charging device for use in an image forming apparatus, such as a copying machine and a printer, and a process-cartridge and an image forming apparatus including such a charging device.
In recent years, in view of advantages, such as a lower ozone-generation characteristic and a lower power consumption, compared with a corona charging device, a contact-scheme charging device (i.e., a contact charging device) including a charging member supplied with a voltage and abutted against an object to be charged for charging the object to be charged has been commercialized.
More specifically, such a contact charging device includes an electroconductive charging member of a roller type (charging roller), a fur brush type, a magnetic brush type or a blade type, and the charging member is caused to contact an object to be charged, such as an image-bearing member, and is supplied with a prescribed bias voltage to charge the surface of the object, to be charged (hereinafter sometimes simply called a xe2x80x9ccharged objectxe2x80x9d or an xe2x80x9cobjectxe2x80x9d) to a prescribed potential of a prescribed polarity.
In contact-charging of an object, two types of charging mechanisms (or charging principles) operate simultaneously, i.e., (1) a discharge-charging mechanism and (2) a direct injection charging mechanism. The characteristics of each contact device are determined depending on which of the two mechanisms is predominant. The two representative charging characteristics (potential-applied voltage characteristics) are represented by curves 70A and 70B in FIG. 7.
(1) A discharge-based Charging Mechanism
This is a mechanism in which the surface of a charged object is charged by electrical discharge occurring across a minute gap between the contact charging member and the charged object. In the discharge-based contact charging system, there is a threshold voltage so that the contact charging member has to be supplied with a voltage larger than the potential level to which the charged object is to be charged. Further, in reality, the occurrence of discharge products and active ions, such as ozone, and difficulties therewith are inevitable in principle, while the amounts of such discharge products are much smaller than in the corona charging device.
Among the known contact charging schemes, a roller charging scheme using a charging roller as the contact charging member is preferred in view of charging stability and has been widely practiced, but the charging mechanism in the roller charging scheme is predominantly governed by the discharge-based charging mechanism.
More specifically, a charging roller is generally formed of an electroconductive or medium-resistivity rubber or foamed material. In some charging rollers, the rubber or foamed layer is included in a laminate structure to obtain a desired property. Such a charging roller is provided with an elasticity so as to obtain a constant contact with the charged object and, as a result thereof, exhibits a large frictional resistance. Accordingly, in many cases, the charging roller is driven so as to follow the movement of or with a relative speed difference with the charged object. Accordingly, the occurrence of a locally non-contact state is inevitable due to the shape irregularity of the roller and the attachment of foreign matter onto the charged object, and as a result, the charging mechanism in the roller charging scheme is dominantly governed by the discharge-based charging scheme.
Referring to a specific example showing a chargeability characteristic as represented by a dashed line 70A in FIG. 7 wherein an organic electrophotographic photosensitive member having a 25 xcexcm-thick photosensitive layer is charged by a charging roller pressed against thereto, the surface potential on the photosensitive member begins to increase when a voltage in excess of, e.g., ca. 500 volts is applied to the charging roller, and at higher applied voltages, the surface potential of the photosensitive member increases linearly at a slope of 1 with respect to the applied voltage. The threshold voltage (of ca. 500 volts on the curve 70A in FIG. 7) may be referred to as a charge initiation voltage (Vth).
Accordingly, in such a roller charging scheme, in order to obtain a surface potential Vd required for an electrophotographic process, it is necessary to apply an additional voltage of Vd+Vth to the charging roller. Such a charging scheme of applying only a DC voltage to a contact charging member and to a charged object may be generally referred to as a xe2x80x9cDC-charging schemexe2x80x9d.
However, according to the DC-charging scheme, it has been difficult to charge the photosensitive member to a constant desired potential value since the resistance of the contact charging member is changed due to changes in environmental conditions, etc., and also Vth is changed due to changes in photosensitive layer thickness of the electrophotographic photosensitive member as the charged object.
For overcoming these difficulties to achieve a further uniform charging scheme, there has been used a so-called xe2x80x9cAC-charging schemexe2x80x9d as disclosed in JP-A 63-149669, wherein a charged object is charged by applying an oscillating voltage obtained by superposing a DC voltage component corresponding to a desired potential Vd with an AC voltage component having a peak-to-peak voltage of at least Vthxc3x972. This scheme utilizes the potential-smoothing effect of AC voltage superposition, and the potential of the charged object is changed to Vd, which is a central value of the oscillating voltage and is less affected by an external change, such as an environmental change.
However, in the above-mentioned contact charging scheme, the essential charging mechanism thereof relies on a discharge from a charging member onto a charged object, and the voltage required for the charging amounts to a value of (photosensitive member surface potential+at least a discharge threshold voltage), thus inevitably generating more or less amounts of discharge products, such as ozone.
Moreover, the AC-charging scheme for uniform charging performance has resulted in other difficulties, such as an increased amount of discharge products such as ozone, vibration noise (AC-charging noise) caused between the contact charging member and the charged object under the application of the AC electric field therebetween, and noticeable surface degradation of the charged object due to the discharge.
(2) Direct Injection Charging Mechanism
This is a charging mechanism as disclosed, e.g., in JP-A 6-3921, wherein charges are directly injected from a contact charging member to a charged object to charge the object.
More specifically, in the direct injection charging scheme, the object is charged with charges directly injected from a medium resistivity contact charging member to the object surface without relying on a discharge phenomenon or discharge mechanism. Accordingly, even at an applied voltage below a discharge threshold voltage, the object can be charged to a potential comparable to the applied potential (an example of a chargeability characteristic (potential-applied voltage characteristic) is represented by a solid line 70B in FIG. 7). The direct injection charging mechanism is substantially free from the occurrence of ions or discharged products and therefore free from the difficulties accompanying it.
More specifically, in such a direct injection charging system, a contact charging member, such as a charging roller, a charging brush or a charging magnetic brush, is supplied with a voltage to inject charges at a trap energy level or to a charge retention member such as electroconductive particles of a charge injection layer. As the discharge phenomenon is not dominant, only a voltage comparable to a surface potential level of a charged object is required to be applied to the charging member. Thus, this method is free from the occurrence of discharge by-products, such as ozone.
Particularly, in the case of a porous roller such as a sponge roller coated with electroconductive fine particles as a contact charging member, it becomes possible to accomplish a very dense contact between the contact charging member and the charged object, thereby easily obtaining good charging performance.
Further, by rotating the charging member with a relative surface speed difference, it becomes possible to obtain a better charging performance by a simple organization of the contact charging device according to the injection charging mechanism, while obviating the occurrence of discharge products, such as ozone.
However, such a contact charging system according to the direct injection charging scheme is still accompanied by the difficulty that a toner, having slipped by a cleaning section, is gradually accumulated on the charging roller to lower the charging performance. Particularly, in, a so-called cleanerless (image forming) system lacking an independent cleaning means for recovering and storing a portion of toner (transfer-residual toner) remaining on a photosensitive member (charged object) after a transfer step and prior to the primary charging by such a contact charging member, wherein the developing means is expected to also function as a cleaning means, the accumulation of the transfer-residual toner is a serious concern, thus rendering this method liable to cause a more frequent lowering in charging performance.
For providing an improvement to the above-mentioned problem, JP-A 2001-188404 has proposed a charging roller having surface concave cells giving a total cell edge perimeter per unit area of 15 mm/mm2 to 60 mm2 so as to improve the charging performance. However, further improvement in charging performance is desired for a long period of the charging operation.
An object of the present invention is to provide a charging device of the contact charging scheme excellent in uniform charging performance and substantially free from the occurrence of discharge by-products, such as ozone.
Other objects of the present invention are to provide an electrophotographic image forming apparatus including a photosensitive member and such a charging device for charging the photosensitive member according to the direct charge-injection charging scheme while retaining a stable charging performance for a long period, thereby providing images free from charging irregularity even in a long period of image formation and a process-cartridge including such a charging device to be incorporated in such an electrophotographic image forming apparatus.
According to the present invention, there is provided a charging device, including: an object to be charged, and a roller-shaped charging member disposed in contact with the object via electroconductive particles and supplied with a voltage to charge the object, the charging member having a surface layer comprising an elastic foam and moved with a surface speed difference relative to the object,
wherein the charging member has surface cavities having an average diameter of 15 to 150 xcexcm and occupies an area percentage of 50 to 90%.
According to the present invention, there is further provided a process-cartridge, comprising an electrophotographic photosensitive member and a charging device including a roller-shaped charging member and integrally supported to form a cartridge which is detachably mountable to a main assembly of an image forming apparatus, wherein
the charging device is disposed in contact with the photosensitive member via electroconductive particles and supplied with a voltage to charge the photosensitive member,
the charging member has a surface layer comprising an elastic foam and is moved with a surface speed difference relative to the photosensitive member,
wherein the charging member has surface cavities having an average diameter of 5 to 150 xcexcm and occupies an area percentage of 50 to 90%.
The present invention further provides an image forming apparatus, comprising an electrophotographic photosensitive member, a charging device including a charging member disposed in contact with the photosensitive member via electroconductive particles and supplied with a voltage to charge the photosensitive member, exposure means for exposing the photosensitive member to light to form an electrostatic latent image on the photosensitive member, developing means for developing the electrostatic latent image with a developer to form a toner image on the photosensitive member, and transfer means for transferring the toner image onto a transfer receiving material, wherein
the charging member has a surface layer comprising an elastic foam and is moved with a surface speed difference relative to the photosensitive member,
wherein the charging member has surface cavities having an average diameter of 5 to 150 xcexcm and occupies an area percentage of 50 to 90%.